function sys=obv(t,x,zt,ut)
sys=zeros(2,1);

L2 = [ -54.8247 63.0280 0 -24.7515;12.3734 -7.8000 0 4.2552];
A = [ 0 1 0 1;0 0 1 0;0 0 0 1;-6 -2.92 -1.2 1];
B = [ 0 0;0 0;1 0;0 1];
D = [ 0.2 0.3;0.2 0.4;0 0;0 0.4];  

dp = - L2*D*x - L2*(D*L2*zt +  A*zt + B*ut);
sys(1) = dp(1);
sys(2) = dp(2);